In recent years, there is a demand for high capacity and reduction in a diameter in memory hard disk drives, and in order to increase recording density, there is a request that a unit recording area be reduced by decreasing a floating amount of a magnetic head. Along with this, requirement for surface quality after polishing is becoming strict year after year also in the step of producing a magnetic disk substrate. That is, it is necessary to reduce surface roughness, micro waviness, roll-off, and protrusions in accordance with reduction in a flying height of a head, and the allowable number of scratches per substrate surface and the allowable size and depth thereof are decreasing along with the reduction in a unit recording area.
Further, integration and speed are increasing also in a semiconductor field, and particularly in high integration, there is a demand that wiring be finer. Consequently, in a method for producing a semiconductor substrate, depth of focus becomes small at a time of exposing a photoresist to light, and hence, further surface smoothness is desired.
In order to reduce scratches formed on a surface of an object for the purpose of improving surface smoothness in response to the above-mentioned request, there has been proposed a polishing composition where the number of coarse particles is reduced by centrifugation and multistage filtration (Patent Documents 1 and 2, for example), and a method for producing the polishing composition. Further, there has been proposed a method for producing a polishing silica sol including a step of passing a silica sol through a filter having a positive zeta potential (Patent Document 3 for example).